Update clang-format includes sorting (#521)

clang-format still thinks that source directories within the project
should start with <isce/

Since the recent layout restructuring renaming "isce" directories to
"isce3", this is no longer accurate.

This commit updates the clang-format config to fix sorting of includes.

.clang-format

@@ -7,15 +7,15 @@ SortIncludes:    true
 # forward.h or main header
 # system headers
 # contrib headers
-# isce headers with angle brackets
+# isce3 headers with angle brackets
 # relative quote includes
 IncludeBlocks:   Regroup
 IncludeCategories:
   - Regex:           '^.*forward\.h'
     Priority:        0
   - Regex:           '^(<|")(cereal|gtest|pyre|Eigen)/'
     Priority:        2
-  - Regex:           '^<isce/'
+  - Regex:           '^<isce3/'
     Priority:        3
   - Regex:           '^"'
     Priority:        4